consolidation.ppt
DESCRIPTION
basic of consolidation in soil mechanicsTRANSCRIPT
• Consolidation:
• Types of settlements:there are two types of settlements.
• They are: • Total settlement(volume change)• Time required for settlement of compressible layer
• It consists of three settlements
• They are• IMMEDIATE SETTLEMENT• PRIMARY CONSOLIDATION SETTLEMENT• SECONDARY CONSOLIDATION SETTLEMENT
TOTAL SETTLEMENT
IMMEDIATE SETTLEMENT
• This settlement occurs immediately after the load is applied. This is due to distortion (change in shape) at constant volume. There is negligible flow of water in less pervious soils.
• In case of pervious soils the flow of water is quick at constant volume. This is determined by elastic theory (E & µ are used).
• It occurs due to expulsion of pore water from the voids of a saturated soil.
• In case of saturated fine grained soils, the deformation is due to squeezing of water from the pores leading to rearrangement of soil particles.
• The movement of pore water depends on the permeability and dissipation of pore water pressure
• This is also called Secondary compression (Creep). “It is the change in volume of a fine grained soil due to rearrangement of soil particles (fabric) at constant effective stress”.
• The rate of secondary consolidation is very slow when compared with primary consolidation.
• Excess Pore water Pressure (∆u)
• “It is the pressure in excess of the equilibrium pore water pressure”. It is represented as ∆u.
» ∆u= hγw
• Where h --- Piezometric head• γw--- Unit weight of water
• Terzaghi’s model consists of a cylindrical vessel with a series of piston separated by springs.
• The space between springs is filled with water the pistons are perforated to allow for passage of water.
Piezometers are inserted at the centers of different compartment to measure the pressure head
• COMPRESSION OF SPRING MASS:
• Terzaghi has correlated the spring mass compression process with the consolidation of saturated clay subjected to external load∆s.
• The springs and the surrounding water represent the saturated soil. The springs represent the soil skeleton networks of soil grains and water in the vessels represents the water in the voids.
• In this arrangement the compression is one dimensional and flow will be in the vertical direction.
• After sometime‘t’ there will be flow of water through perforation beginning from upper compartment. In the lower compartment the volume of water remains constant
• Due to flow of water in the upper segment there will be reduction in volume due to this springs get compressed .
• They being to carry a portion of the applied load
• Soil Compressibility
Void ratio-effective stress and compression-time plots for sand
Sand deposit compresses immediately on load application. Loose sandcompresses more than dense sand. Loose and dense sand deposits tend
• Compression of fine grained soil (Clay)
Void ratio-effective stress and compression-time plots for clay
Time dependent compression takes longer time compared to sand. The magnitude of compression is also large.
• Assumptions:
– The soil medium is completely saturated– The soil medium is isotropic and homogeneous– Darcy’s law is valid for flow of water– Flow is one dimensional in the vertical direction– The coefficient of permeability is constant– The coefficient of volume compressibility is constant– The increase in stress on the compressible soil
deposit is constant .– Soil particles and water are incompressible
• One dimensional theory is based on the following hypothesis
– 1. The change in volume of soil is equal to volume of pore water expelled.
– 2. The volume of pore water expelled is equal to change in volume of voids.
– 3. Since compression is in one direction the change in volume is equal to change in height.
• Uniform excess pore water pressure with depth – 1. Single Drainage (Drainage at top and bottom impervious)
– 2. Double Drainage (Drainage at top and bottom)
Single Drainage (drainage at top and bottom impervious) :
Excess porewater pressure distribution of single drainage
• Double Drainage
Excess porewater pressure distribution of double drainage
• Degree of Consolidation:
a) Section of clay layer, (b) Excess pore pressure distribution